Projection-type display devices using liquid crystal light valves become highly popular because of their compact size and light weight as compared to conventional projection-type display devices using cathode ray tubes. Many different liquid crystal light valves have been presented for improving gray scale, color reproducibility, and color purity as well as resolution and contrast ratio. As an example, U.S. Pat. No. 4,995,702 issued to Aruga shows a projection-type display device using three twisted nematic liquid crystal light valves. The product of the birefringence and the thickness of the liquid crystal cell is selected to fall within a predetermined optimal range of values dependent on the wavelength of light which travels through the light valve.
Liquid crystal light valves have been used in both reflection-type and transmission-type projection display. In general, high efficiency, high contrast, low color difference and low driving voltage are four important requirements for the liquid crystal valves used in projection displays. Twisted nematic (TN) liquid crystal cells have been widely used in transmission-mode active matrix liquid crystal displays of high quality. However, when a transmission-mode TN cell is used in reflective-mode display, it results in poor brightness and low resolution.
U.S. Pat. No. 5,105,289 issued to Sonehara et al. describes an improved reflection-type projection display with liquid crystal electro-optical device by optimizing the twist angle, the product of layer thickness and birefringence of the liquid crystal cell. Mixed-mode twisted nematic (MTN) liquid crystal cells for reflection-type projection displays have also been proposed by Wu et al. lately in (1) ixed-mode twisted nematic liquid crystal cells for reflective displays" of Applied Physics Letter Vol. 68, No. 11, pp. 1455-1457, 11 Mar. 1996, (2) igh-Brightness Projection Displays Using Mixed-Mode Twisted-Nematic Liquid-Crystal Cells" of SID 96 Digest, pp. 763-766, and (3) igh Brightness Liquid Crystal Projection Displays" of Japan Journal of Applied Physics, Vol. 35, No. 10, pp. 5349-5354, October 1996.
The structure and fabrication process of the MTN cell are nearly identical to a conventional 90.degree. TN cell except for two major differences. One is that MTN cell has a smaller product of birefringence and thickness than a nominal TN cell. The other 2 is that the front director of the MTN cell must be aligned at an angle to the polarization state of the incident light. In Wu prior arts (2) and (3), mixed mode chiral-homeotropic cells (or inverse TN cells) are introduced for reflective-mode displays. In a voltage-off state, the liquid crystal directors are aligned perpendicular to substrates in spite of the existence of the chiral molecules. In a voltage-on state, they tend to follow the twist of the chiral molecules and act as a TN cell. The electro-optic effects of a reflective mixed-mode chiral-homeotropic cell is equivalent to an inverse MTN cell.